1. Field of the Invention
The present invention relates to an ink jet head for ejecting ink droplets from nozzles each communicating with each of plural ink chambers formed in a cavity plate, on a recording sheet to record images such as characters, by applying a driving voltage to piezoelectric elements disposed with respect to the ink chambers respectively, thereby pressurizing the ink chambers. More particularly, the present invention relates to an ink jet head capable of making the ink ejection velocity uniform within a predetermined range regardless of the number of ink chambers to be pressurized at the same time through corresponding piezoelectric elements by setting a predetermined correlation between the pulse width of a driving voltage to be applied to piezoelectric elements and the time necessary for travelling of a pressure wave of ink through the ink chamber by a length thereof, the pressure wave generating when the piezoelectric element is pressurized.
2. Description of Related Art
Conventionally, with regard to ink jet heads, there have been studied on the correlation between the ejection velocity of ink droplet and the pulse width of a driving voltage applied to piezoelectric elements at the time of ejecting of ink droplets from nozzles communicating with corresponding ink chambers by applying a driving voltage to the piezoelectric elements each disposed correspondingly with each of the ink chambers. In general, it is known that the ejection velocity of ink droplets when ejected from nozzles varies periodically according to the pulse width of a driving voltage applied to piezoelectric elements.
For example, in the case that driving voltages having different pulse widths are applied to a piezoelectric element, pressurizing an ink chamber of an ink jet head thereby to eject ink droplets from a nozzle, such relationship as indicated by a curved line S1 (solid line) in FIG. 5 is seen between the ejection velocity of ink droplets and the pulse width of the driving voltage. In this case, if assuming "T" to be the time necessary for travelling of a pressure wave of ink by a length of the ink chamber, the pulse width of a driving voltage corresponds to the time T at the first peak point K1 (the left peak in FIG. 5) in the curved line S1, and the other pulse width of a driving voltage corresponds to the time 3T being three times the time T at the second peak point K2 (the right peak in FIG. 5).
In ink jet heads in the prior art, based on the correlation between the ink ejection velocity and the driving voltage applied to the piezoelectric element mentioned as above, the pulse width of a driving voltage for driving each piezoelectric element has been determined in consideration of the case of pressurizing plural ink chambers at the same time.
Meanwhile, the time T necessary for a pressure wave of ink, which generates when an ink chamber is pressurized, traveling by a length of the ink chamber is determined as follows; EQU T=L/.sqroot.0(E.sub.v /.rho.)
where L indicates the length of an ink chamber, E.sub.v indicates apparent modulus of elasticity of volume of ink, and .rho. is the density of ink. This modulus of elasticity of ink volume E.sub.v has a property of varying according to the deformed amount of an ink chamber when pressurized and decreasing as the deformed amount of an ink chamber increases.
In the ink jet head, naturally, there is also the case of pressurizing a plurality of ink chambers at the same time, in addition to the above mentioned case of pressurizing only one of ink chambers in an ink jet head. Here, as an example opposite to the above case where an ink chamber alone is pressurized, the case where all ink chambers are pressurized at the same time will be reviewed below.
When all ink chambers are pressurized at the same time through respective corresponding piezoelectric elements, a cavity plate in which each ink chamber is formed may be deflected and deformed in a pressurized direction. The deflected and deformed amount of the cavity plate expectedly becomes larger as compared with the case of one ink chamber alone pressurized, so that the ink apparent volume elasticity modulus E.sub.v in each ink chamber becomes small as mentioned above. As is clear from the above formula, as the volume modulus of elasticity of ink E.sub.v becomes small, the time T becomes large. The ejection velocity of ink droplets ejected from nozzles when all ink chambers are pressurized at the same time changes periodically according to the width of a driving voltage applied to a piezoelectric element, when the curved line is entirely shifted in a side of larger pulse width. Specifically, as a curved line S2 (broken line) in FIG. 5, the pulse width of the driving voltage that may produce the same ink ejection velocity as in the curved line S1 is entirely shifted rightward in the graph, and both peak points K3 (the left peak in the graph) and K4 (the right peak) are also shifted rightward.
Consequently, as shown in FIG. 5, if the pulse width of a driving voltage being to be applied to a piezoelectric element is set to P.sub.A, for instance, the velocity of ink droplet ejection when only one ink chamber is pressurized is V.sub.A, while the velocity of ink droplet ejection when all ink chambers are pressurized is reduced to V.sub.B. Regarding the ink droplet ejection velocity V.sub.A and V.sub.B, it is necessary that both the velocities V.sub.A and V.sub.B are values more than a fixed value and also a difference between V.sub.A and V.sub.B is within a predetermined allowable range. If the V.sub.A and V.sub.B are not more than a fixed value and the difference is out of the predetermined allowable range, the ink droplet ejection velocity V.sub.A and V.sub.B may be changed depending on the number of ink chambers pressurized, causing deterioration in the printing quality.